The field of digital data compression and in particular digital image compression has attracted a great interest for some time.
The compression of images using a lossy format requires a number of decisions to be made. Firstly, the resolution at which the image is to be stored must be determined. This is usually driven by the application of the image and it is common to save using a number of different resolutions for different applications such as on-screen display, web publication, printing, previewing etc. Secondly there is a choice as to the quality or amount of loss that will be permitted. This choice directly affects the compression rate for the image and, so, is often application driven as well. Finally the progression order of the codestream may be important to the particular application. This order can affect how an image is progressively updated as it is transferred over a slow network connection such as the Internet.
A widely used standard for image compression is the “Joint Photographic Experts Group” or JPEG standard for image compression. A new image compression method herein after called JPEG2000 has recently been standardized by the ISO (ISO/IEC 15444-1:2000-Part I of JPEG2000). Part 11 of JPEG2000 is currently in the process of being standardized by the JPEG group and a draft standard has been published entitled “INFORMATION TECHNOLOGY—JPEG2000 IMAGE CODING SYSTEM: EXTENSIONS—JPEG2000 Part 11 Final Draft International Standard, Study Document Pre-Release B, 9 Mar. 2001”. JPEG2000 (part 1) is an image compression standard that offers excellent compression and many features. One of the main features of JPEG2000 (part 1) is progressive transmission. JPEG2000 (part 1) supports a number of different progressive modes including progressive by quality and progressive by resolution. A JPEG2000 decoder can extract or receive a subset of the compressed image codestream, and use this to reconstruct a lower resolution or lower quality image.
The problem with older standards such as the previous JPEG standard was that these decisions concerning different resolutions had to be made at encode time. This meant that, often, many different versions of each image would need to be generated. One to serve as an original and numerous application specific versions. The JPEG2000 digital image compression standard improves on this situation by deferring these decisions to decode time. When using a JPEG2000 compressed image it is theoretically possible to store a single “original” and decode it differently for each application. For this reason, much is made of the potential applications of JPEG2000 in client server environments such as the Internet. In practice however, JPEG2000 does not in itself provide a solution to this problem, rather it is a tool which may be used in many different ways to achieve any given task.